The present invention relates to an anti-pressure surge device mounted on or in the sealing element of a check especially a dual plate check valve.
Check valves of known types such as tilt disk check valves, swing check valves and dual plate check valves are well known in the art and are used in a pipe line to allow flow to go in one direction called the forward flow direction but to automatically shut if flow in the line reverses. The shutting action is effected by the line fluid acting on the closure member generally termed the plate of the check valve. Once the closure member of the valve is seated in the closed position no further reverse flow can take place.
Pipelines are designed to a given pressure class rating. The higher the pressure class rating the thicker the walls of the components in the pipeline have to be to withstand that pressure. The effect of closure of check valves can however necessitate the pressure class of the pipeline being increased with a substantial increase in the cost of the pipeline in order to accommodate surges in pressure associated with the closure of the check valves. Different types of check valves close at different rates for the same line condition. When reverse flow begins, causing the check valve to close, reverse velocity of fluid in the pipe is built up in the time taken for the valve to close. Once closed, the continued momentum of the fluid causes a downstream pressure peak considerably above the original line pressure which may then oscillate with decaying peaks above the original line pressure until the line pressure stabilises. It is these peaks that give the piping engineer a pressure class rating problem and with the associated water hammer gives the piping engineer a pipework foundation problem.
Typically check valves would find application in a system where the fluid was being pumped in a direction (the downstream direction) against a system deceleration, in other words a deceleration that would be experienced when the pump is turned off. The check valves are intended to prevent the system deceleration from causing the fluid to return in the upstream direction if pump power is turned off or lost. Any check valve will take a period of time to close. The time in which the check valve closes when exposed to a given system deceleration when reverse flow occurs is known as the response time. If the check valve has a fast response time to a reverse system deceleration, then only a low reverse flow velocity (Vr max) will be reached by the time the check valve has closed. Conversely if the check valve has a slow response time then a high reverse flow velocity will be reached by the time the check valve has closed. The Vr max may in itself be a problem to pumps being rotated backwards for a short time, but the more serious problem is the instantaneous halting of the column of fluid that occurs when the check valve member does close. The weight of the fluid column on large valves can be many tons. This force, resulting from the change of kinetic energy of the fluid being brought to an abrupt halt by the closed check valve, is transmitted through the check valve to the adjacent pipework. The resultant forces on foundations can be very severe. The instantaneous halt of the column of liquid when a check valve closes may be considered to give rise to a pipeline pressure increase according to the Joukowsky formula:
Pressure rise=Speed of sound in the fluid linexc3x97(Vr max)xc3x97the fluid density
The problem facing piping engineers when they have a naturally high system deceleration and when the check valve has a moderate or slow response combining to provide a high Vr max is the potential of the line pressure rising excessively due to the Joukowsky pressure surge. Furthermore the high Vr max can also lead to severe vibration of pipework foundations due to the water hammer effect.
A pipeline designer worried about over pressure may choose to specify a higher pipeline pressure rating, but at a considerable additional cost, or may choose to specify a check valve which minimises the reverse pressure effect. The reverse pressure effect can be minimised by selecting a valve which closes more rapidly so allowing less time for build up of reverse velocity. Thus the designer may choose a nozzle valve in preference to a dual plate check valve, swing check valve or a tilting disk check valve. However the nozzle valve does require a wider body to achieve comparable flow characteristics to a dual check valve and it may not always be suitable. Moreover the cost of a nozzle valve may be at least four times that of a dual plate check valve.
Swing check valves due to the distance that the closure member has to travel can seldom be made to close quickly enough for systems with high dv/dt. One solution is to attach a damping mechanism to the closure member to prevent slam, that is to say dramatically to slow down speed of closure over the final distance of travel, or fit a bypass loop around the check valve. This in part defeats the purpose of the check valve. The damping mechanisms are difficult to set up and maintain. Also the associate reverse flow through pumps and compressors must be acceptable which may be a particular problem with multiple pumps in parallel.
Dual plate check valves have commonly been in use for over 35 years. They are manufactured by over 40 companies throughout the world. The dual plate check valve closes much faster than the swing check valve, but does not close as fast as the nozzle valve. For applications where there is a risk of rapid reverse flow occurring, pipeline specifiers often resort to the much more expensive nozzle valve due to potential problems of over pressure. Additionally or alternatively the specifiers may specify a pressure relief pipe loop which has the disadvantage of breaking the pressure boundary provided by the pipe. Fitting a dual plate check valve with a stronger spring may help to increase the speed of closure but also increases the critical velocity. If the specified velocity is above the natural pipeline flow rate, the plates of the valve never fully open and the pressure drop across the valve is high.
JP 10213240 discloses a dual plate check valve where each plate is fitted with two sub-valves. The first sub-valve can be opened so as to discharge pressure to the outlet side without opening the main plate when there is a modest flow in the downstream direction. This is said to prevent chattering at low flow rates which can cause wear and tear. The second sub-valve opens when excessive pressure from the outlet side occurs to let pressure off to the inlet side. The second sub-valve is described as providing pressure relief to avoid damage to valve components. The flow paths of the second sub-valve as depicted would enable the second sub-valve to perform the function of relieving excess pressure but would not be expected to limit the surge effect on valve closure. Thus a system using a check valve in accordance with JP 10213240 would be expected to experience the full Joukowsky pressure effect on closure of the check valve.
According to the present invention there is provided a high flow rate anti-pressure surge device mounted, preferably removably, in or on the closure member of a dual plate, tilting disk or swing check valve wherein the anti-pressure surge device comprises a pressure sensitive valve which allows flow in an upstream or reverse direction when subjected to a reverse pressure differential exceeding a predetermined limit. Such a device can substantially reduce the level and/or duration of pressure peaks when the valve closes and can also suppress pressure oscillations in the pipeline. This has the effect of avoiding the need for the pipeline engineer to have to upgrade the pressure class of the pipeline.
In a preferred embodiment the anti-pressure surge device operates by the effect of the back pressure shock wave acting on a piston face which at a certain excess pressure of the downstream side coupled with the drop of pressure on the upstream side allows the piston to move against an appropriate resilient force such as a spring or hydraulic force to allow flow to pass backwards through the one way valve with a flow rate sufficient to reduce the peak pressure below the Joukowsky pressure which would arise in the absence of the device and/or to dissipate some of the energy associated with the Joukowsky pressure which would arise in the absence of the device.
The invention will be more clearly described by way of example only by reference to the attached figures in which: